In known manner, the leakage rate is calculated from the density of the gas contained inside the envelope, where density is itself determined by a temperature-compensated pressure sensor. It should be observed that compensation can also be performed by a temperature sensor that is distinct from the pressure sensor. Nevertheless, a single sensor simplifies assembly.
The temperature compensated pressure sensor must be fixed to the outside wall of the envelope in order to avoid the risk of an electric arc being stuck between the envelope and the circuit breaker under the effect of the high voltage, and it must have a pressure detector and a temperature detector that are in communication with the gas via a duct passing through the thickness of the envelope.
The temperature as measured must be as close as possible to the temperature of the gas. FIG. 1 shows how the density .rho. of the gas varies as a function of temperature T for a given pressure P. At a temperature T, the gas has density .rho.. If the sensor picks up a temperature T+.DELTA.T that differs from T by uncertainty .DELTA.T (which is positive in the example of FIG. 1), then the compensation performed on the pressure P will lead to a density .rho.-.DELTA..rho. that is erroneous having uncertainty .DELTA..rho. (which is negative in the example of FIG. 1).
To detect and track over time the evolution of any leak of gas from the envelope of the circuit breaker, the measurement uncertainty associated with the density sensor must be negligible compared with any reduction of density due to a leak.
An annual leakage rate of the order of 1% is assumed during testing performed on site. Although the sensor used has density measurement stability of the order of 0.2% over a year, differences of several percent have been observed between daily averages of density.
Tests have shown that sunshine and heating of the electrical apparatus constitute two major sources of uncertainty in determining density. As a function of the amount of sunshine and of the electrical current flowing through the apparatus, the envelope is subjected to daily variations of temperature. These variations give rise to temperature differences between the gas and the detector, since they response differently to the thermal influence of the envelope at the communication duct formed through its thickness.